Drug Interactions between amitriptyline / perphenazine and methadone

MONITOR CLOSELY: Concomitant use of methadone with other central nervous system (CNS) depressants (e.g., sedatives/hypnotics, anxiolytics, muscle relaxants, antipsychotics, or tricyclic antidepressants) may result in profound sedation, respiratory depression, coma, and death. In addition, concomitant use of methadone with agents that possess or enhance serotonergic activity may potentiate the risk of a potentially life-threatening condition called serotonin syndrome. The exact mechanism by which serotonin syndrome occurs may differ based on the agent(s) involved. It is thought to result from the hyperstimulation of postsynaptic 5-HT receptors and while postsynaptic 5-HT1A and 5-HT2A receptors are usually implicated, it is more likely that no single receptor is solely responsible. Symptoms of the serotonin syndrome may include mental status changes such as irritability, altered consciousness, confusion, hallucination, and coma; autonomic dysfunction such as tachycardia, hyperthermia, diaphoresis, shivering, blood pressure lability, and mydriasis; neuromuscular abnormalities such as hyperreflexia, myoclonus, tremor, rigidity, and ataxia; and gastrointestinal symptoms such as abdominal cramping, nausea, vomiting, and diarrhea.

MONITOR CLOSELY: Methadone can cause dose-related prolongation of the QT interval. Coadministration with other agents that can prolong the QT interval may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. High dosages of methadone alone have been associated with QT interval prolongation and torsade de pointes. In a retrospective study of 17 methadone-treated patients who developed torsade de pointes, the mean daily dose was approximately 400 mg (range 65 to 1000 mg) and the mean corrected QT (QTc) interval on presentation was 615 msec. The daily methadone dose correlated positively with the QTc interval. Fourteen patients had at least one predisposing risk factor for arrhythmia (hypokalemia; hypomagnesemia; concomitant use of a medication known to prolong the QT interval or inhibit the metabolism of methadone; and structural heart disease), but these were not predictive of QTc interval. It is not known whether any of the patients had congenital long QT syndrome.

MANAGEMENT: The use of methadone with other agents that are CNS depressants and/or prolong the QT interval and/or possess or enhance serotonergic activity should generally be avoided unless alternative treatment options are inadequate. If coadministration is necessary, the dosage and duration of each drug should be limited to the minimum required to achieve desired clinical effect, with cautious titration and dosage adjustments when needed. Patients should be monitored closely for and counseled about the signs and symptoms of serotonin syndrome (e.g., altered mental status, hypertension, restlessness, myoclonus, hyperthermia, hyperreflexia, diaphoresis, shivering, and tremor) as well as prolonged QT interval (irregular heartbeat, dizziness, lightheadedness, fainting) and respiratory depression and sedation, and advised to avoid driving or operating hazardous machinery until they know how these medications affect them.

MONITOR CLOSELY: Methadone can cause dose-related prolongation of the QT interval. Theoretically, coadministration with other agents that can prolong the QT interval may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. High dosages of methadone alone have been associated with QT interval prolongation and torsade de pointes. In a retrospective study of 17 methadone-treated patients who developed torsade de pointes, the mean daily dose was approximately 400 mg (range 65 to 1000 mg) and the mean corrected QT (QTc) interval on presentation was 615 msec. The daily methadone dose correlated positively with the QTc interval. Fourteen patients had at least one predisposing risk factor for arrhythmia (hypokalemia, hypomagnesemia, concomitant use of a medication known to prolong the QT interval or inhibit the metabolism of methadone, and structural heart disease), but these were not predictive of QTc interval. It is not known if any of the patients had congenital long QT syndrome.

MANAGEMENT: Caution is recommended if methadone is used in combination with other drugs that can prolong the QT interval, particularly in the setting of chronic pain management or methadone maintenance for opioid dependency where high dosages may be employed. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope. If taking drugs that also cause CNS-depressant or orthostatic effects (e.g., sedating antihistamines, psychotropic drugs like tricyclic antidepressants, phenothiazines, and neuroleptics), patients should be made aware of the possibility of additive effects with methadone and counseled to avoid activities requiring mental alertness until they know how these agents affect them.

MONITOR: Coadministration of a phenothiazine with a tricyclic antidepressant (TCA) may result in elevated plasma concentrations of one or both drugs as well as additive adverse effects. Most phenothiazines and TCAs have been found to undergo metabolism by CYP450 2D6, thus competitive inhibition of the enzyme may occur when more than one of these agents are administered. Although these drugs have been used together clinically, the possibility of increased risk of serious adverse effects such as central nervous system depression, tardive dyskinesia, hypotension, and prolongation of the QT interval should be considered, as many of these agents alone can and have produced these effects. In addition, excessive anticholinergic effects may occur in combination use, which can result in paralytic ileus, hyperthermia, heat stroke, and the anticholinergic intoxication syndrome. Peripheral symptoms of anticholinergic intoxication commonly include mydriasis, blurred vision, flushed face, fever, dry skin and mucous membranes, tachycardia, urinary retention, and constipation. Central symptoms may include memory loss, disorientation, incoherence, hallucinations, psychosis, delirium, hyperactivity, twitching or jerking movements, stereotypy, and seizures.

MANAGEMENT: Concurrent use of phenothiazines and TCAs should be approached with caution, particularly in the elderly and those with underlying organic brain disease, who tend to be more sensitive to the central anticholinergic effects of these drugs and in whom toxicity symptoms may be easily overlooked. Patients should be advised to notify their physician promptly if they experience potential symptoms of anticholinergic intoxication (e.g., abdominal pain, fever, heat intolerance, blurred vision, confusion, hallucinations) or cardiovascular toxicity (e.g., dizziness, palpitations, arrhythmias, syncope). Ambulatory patients should be counseled to avoid activities requiring mental alertness until they know how these agents affect them. A dosage reduction in one or both drugs may be necessary if excessive adverse effects develop.